1. Field of the Invention
The present invention relates to a sensing device for an acceleration sensor which is used for detecting a dynamic transition state, i.e., an acceleration state or a deceleration state of an object.
2. Description of the Related Art
Generally, an acceleration sensor detects a dynamic transition state such as an acceleration state or a deceleration state of an object. Therefore, the acceleration sensor can be used for measuring a vibration of the object. For example, in an automobile, the acceleration sensor can be used in the determination of a collision to activate an airbag. Also, the velocity of an object can be measured by integrating the output of the acceleration sensor, and in addition, the displacement of the object can be measured by integrating such a velocity. Therefore, the acceleration sensor can be also used in a transition control system which is for example, used for increasing or decreasing a fuel injection amount in an automobile.
As an acceleration sensor, a semiconductor acceleration sensor using the piezoresistance effect, which is small in size, light in weight, has a large anti-impact characteristic, and the like, is well known (see: Japanese Unexamined Patent Publication (Kokai) No.Sho 63-255664). The semiconductor acceleration sensor uses the strong mechanical strength of silicon and fine lithography technology applied thereto. The semiconductor acceleration sensor mainly includes a weight for receiving an acceleration force (inertia force), a cantilever having a stationary end and a free end for converting the acceleration force into a stress, and an element for converting this stress into an electrical signal. For example, the element for converting the stress into the electrical signal is constructed with a piezoresistance element by forming a diffusion region within a monocrystalline silicon substrate. In this case, in order to maximize a stress sensitivity change of the piezoresistance element, a bridge circuit is constructed having the piezoresistance element in at least one edge of the bridge circuit. Usually, since the voltage amplitude of the output signals of a bridge circuit is too small to be supplied to a circuit such as an interface of a microcomputer, the voltage amplitude is amplified by an amplifier, and accordingly, this voltage amplitude is on the order of some volts. In this case, the voltage amplification factor of such an amplifier is very large, for example, from 10 to 100,000. When the voltage amplification factor of the amplifier is large, an offset voltage due to the unbalance of the bridge circuit (i.e., the fluctuation of the piezoresistance element) and a thermal drift (i.e., a fluctuation of the offset voltage) strongly affects the output of the amplifier, and accordingly, it is impossible to ensure the accuracy of a measured acceleration within a wide range of temperature. At worst, a large offset voltage generated in the bridge circuit saturates the operation of the amplifier to disable the amplifier.
To ensure operation over a wide temperature range, a prior art acceleration sensing device for the above-mentioned semiconductor acceleration sensor includes a differential amplifier which has a small amplification factor, a high pass filter for removing a DC component from an output of the differential amplifier, and a main amplifier having a large amplification factor (see: FIG. 3 of Kokai No.Sho 63-255664). That is, this acceleration sensing device has a two-stage amplification configuration to ensure a wide temperature range operation, which will be explained later in detail.
In the above-mentioned prior art acceleration sensing device, however, since the high pass filter includes a capacitor, the phase and amplitude of a sensing signal are dependent upon the frequency thereof, and accordingly, it is impossible to ensure accurate acceleration sensing. At worst, when the phase of a sensing signal is too changed, a positive acceleration may be recognized as a negative acceleration (i.e., deceleration), or vice versa. Also, when the amplitude of the sensing signal is changed, it is impossible to measure an absolute value of acceleration.
In addition, in the above-mentioned prior art device, the capacitor of the high pass filter is connected in series to the main amplifer which may be constructed by an operational amplifier having a feedback resistor. Therefore, a bias current, which may enable the main amplifier to be operated stably, is not supplied to the main amplifier, and as a result, the main amplifier cannot operate normally, which also does not ensure an accurate acceleration sensing.
Further, in the above-mentioned prior art device, the capacity of the capacitor of the high pass filter has to be large to reduce the output impedance thereof. Otherwise, a noise characteristic of the high pass filter is inhibited.
Still further, in the above-mentioned prior art, although there is no offset effect in the output signal of the high pass filter due to the differential amplifier and the capacitor of the high pass filter, an offset voltage may be generated in the output signal of the main amplifier due to the fact that the main amplifier has an inherent offset voltage. Therefore, the main amplifier requires a good offset characteristic, which increases the manufacturing cost of the acceleration sensing device.
Further, since the capacitor is interposed serially in the prior art device, this device generates very little acceleration output showing an absolute value of the acceleration force. That is, this device is suitably used only for measuring an absolute value of acceleration or deceleration, a vibration of an object which has a relatively high frequency. In other words, this device is not suitable for measuring a velocity, a displacement, and the like, of an object, i.e., this device is not suitable for a transition control system which is, for example, used for increasing or decreasing a fuel injection amount when acceleration or deceleration occurs.